Does Male Competition Help Explain Human Sex Differences in Body Size?

Anthropologist Holly Dunsworth has a new paper questioning the common emphasis placed on sexual selection for explaining sex differences in human height. She noted on Twitter that the research for her paper was kick-started by a disagreement she had with evolutionary biologist Jerry Coyne on the same topic a few years ago. Jerry Coyne’s post on the matter can be found here.

To start with, I will say that, despite disagreeing with her broader argument, I think there are two important contributions Dunsworth makes in her paper.

First, Dunsworth emphasizes the importance of considering developmental factors for understanding the emergence of physical sex differences, and discuses the role steroid hormones like estrogen play in long bone growth. I found this section interesting and think exploring how developmental factors contribute to physical sex differences is a valuable line of research.

Second, Dunsworth appropriately notes that certain sex differences in body size is an ancestral condition shared with nonhuman apes. Dunsworth writes,

Great apes develop sex differences in body mass like humans do, where both sexes follow similar growth trajectories until the pubertal transition when the females stop growing and the males continue to grow for a longer period of time. Though levels of sex differences in body size differ between species, among the living hominids (great apes and humans) there is likely to be significant shared fundamental biology of reproduction and skeletal growth. Thus, the existence of human sex differences in stature is rooted in ancestry.

I think this is a valuable point, and in fact, as I will explain, I think this idea actually helps explain why both Coyne and Dunsworth come to, in my view, somewhat misleading and/or premature conclusions.

Now, if Dunsworth were to focus entirely on the role of sexual selection for explaining sex differences in human height alone, I think the reference to ancestral patterns would raise some useful questions about the necessity of a sexual selection explanation, as it could instead represent a largely conserved ancestral pattern that did not necessarily require sexual selection on human males or more recent human ancestors per se (more on this later). However, Dunsworth wishes to extend her critique across other primate species, writing that,

Yet even within this more complete “ultimate” narrative, with selection optimizing the two sexes' skeletal growth separately, the sexual selection perspective on male height seems unnecessary. That provocative last sentence is not a claim that the sexual selection explanation is wrong or that it is implausible. But in light of what is known and still unknown about skeletal development and its relationship to the endocrinology of reproduction, suddenly there is room for skepticism about the relevance of male competition and female choice as an explanation for the existence of sex differences in stature, let alone its singular dominance of the narrative. More work is needed if sexual selection is to be held up as the explanation for why male hominids [great apes] have longer bones than female hominids do.

I think it is important not to conflate ‘male competition’ and ‘female choice’ as explanations—even if there may be overlap in some cases, they can be quite distinct. I will focus here on male competition specifically (Dunsworth also usually refers to the male competition explanation in the paper, despite occasionally mentioning female choice). Dunsworth writes that,

the traditional and enduring textbook explanation for sex differences in hominid body size is sexual selection—with large ancestral males winning competitions, which boosted their reproductive success compared to smaller males. Because gorillas have both intense male competition and large male bodies, the mere existence of sex differences in human body size serves as evidence of sexual selection being the driver of these differences. But as Plavcan has cautioned, there is not a straight-forward relationship between sexual selection and primate male body size, largely because the sorts of data that are required to investigate this relationship are difficult to obtain.

Unfortunately, I think this reference to Plavcan’s work ends up being quite misleading. The two papers that Dunsworth cites here are Plavcan (2001) and Plavcan (2012). I think both papers severely undercut her argument rather than bolstering it.

In his conclusion, Plavcan (2001) writes,

It is now apparent that sexual selection, mating systems, and estimates of the frequency and intensity of male-male competition do not measure the same thing. This leads us to the conclusion that sexual dimorphism is not a simple function of differential male reproductive success, but also of pressures on males for display, of the interaction between the utility of different traits as weapons and different fighting strategies at different sizes and in different  habitats, and of female counterstrategies against male domination of mating patterns. Dimorphism is also a function of different developmental pathways, all of which can be affected by different selective pressures throughout an animal’s entire life-history. Understanding these pathways and the selective forces that mold them gives profound insights into the evolution of dimorphism.

Notably while Plavcan, like Dunsworth, gives appropriate credit to the role of developmental pathways and multiple contributing selection pressures for understanding patterns of sexual dimorphism across primates, he, unlike Dunsworth, very strongly emphasizes the role of (multiple forms of) male competition in contributing to these patterns, for reasons that are apparent in the data he provides. For example, consider this table:

From Plavcan (2001).

As can be seen here, and as he discusses in the paper, there is a clear relationship between body mass dimorphism and canine dimorphism. This pattern seems very difficult to explain without male competition playing a significant role in explaining size dimorphism across primates, considering the importance of canines as weapons and intrasexual signaling in conflict across primates.

Notably, humans also still have some sexual dimorphism in canine height, comparatively small though it is.

From ‘The Evolution of the Human Mating System’ by Muller & Pilbeam, in Chimpanzees and Human Evolution (2017) edited by Muller, Wrangham & Pilbeam.

Since in humans canines no longer function as useful weapons or for signaling purposes, this may actually bolster the case that some aspects of human size dimorphism represent a conserved ancestral pattern (perhaps a developmental suite of features) that need not necessarily be due to sexual selection in humans or more recent human ancestors specifically (contra Coyne’s direct focus on this point). However, it conflicts with Dunsworth’s de-emphasis on male competition across hominids more broadly.

Plavcan (2012) is even more explicit in emphasizing male competition and disagreeing with the account offered by Dunsworth. He says it very clearly,

There is no species of primate that shows strong size dimorphism without male competition and polygyny. Two cercopithecoid taxa were thought to show a combination of strong size dimorphism and monogamy (Cercopithecus neglectus and Simias concolor; Leutennegger and Lubach 1987). Further study of both of these species demonstrated that polygyny is present in other parts of the species range and that hunting pressure explained the presence of monogamous pairs at least in S. concolor (Brennan 1985; Watanabe 1981).On the other hand, there are no monogamous or polyandrous anthropoids that show strong size dimorphism, though several monogamous species do show modest levels of size dimorphism (Plavcan 2000a, 2001). If selection to alter female size—either larger or smaller—is common independent of changes in male size and contributes significantly to inter-specific variation in size dimorphism, then we should expect to see greater variation in dimorphism among monogamous and polyandrous species. That this is not the case suggests that large male size in dimorphic species is maintained in spite of pressure to reduce male size to that of females and that most substantial dimorphism in primates is a function of changes in male and not female size [emphasis added].

In a more recent article, Plavcan (2018) even notes that while, “Modern humans are characterized by a modest degree of size dimorphism—about 15 percent on average across populations,” and that “This magnitude of dimorphism falls within that seen in modern gibbons and is less than that of chimpanzees,” he also mentions that, “modern human males are substantially more muscular than females, and “lean-mass” dimorphism is comparable to the size dimorphism of chimpanzees,” perhaps pointing to how focusing on simple metrics of size may obscure other important sex differences relevant to male competition.

Now, while I do think it is quite uncertain what role sexual selection in humans and recent human ancestors specifically may have played in influencing patterns of sexual dimorphism, there is a great deal of research indicating a significant relationship among human males between status and height/body size, consistent with broader patterns in primates as well. ‘Big man’ is an extremely common designation for a leader across small-scale societies and even some nation states. For example, the Sumerian term for ‘king’, Lugal, is GAL+LU = BIG plus MAN. Across nonindustrial societies larger and higher status men tend to have greater reproductive success, although importantly this relationship seems to be substantially smaller (r = 0.19) than that found across nonhuman primates (r = 0.80).

“Comparing weighted effect sizes of men’s status on measures of RS, from the model with subsistence as the only covariate, with effects of male dominance rank on mating success in nonhuman primates (16). Minimal variation was found across subsistence types, yet as a group, humans have significantly lower effects of male status on reproduction compared with nonhuman primates. Point size and line width are proportional to the number of results contributing to each weighted effect size.” From von Rueden & Jaeggi (2016). Men’s status and reproductive success in 33 nonindustrial societies: Effects of subsistence, marriage system, and reproductive strategy. PNAS

Similarly, Dunsworth’s de-emphasis on male competition more generally makes certain common cross-cultural patterns difficult to understand, particularly in light of men’s overrepresentation in committing lethal violence and participating in warfare. As I discussed in my review for Human Nature of Nam C. Kim and Marc Kissel’s book Emergent Warfare in our Evolutionary Past,

Sex differences in participation in warfare are widely documented (Goldstein 2001) and are clearly illustrated by male-biased casualty rates (Beckerman and Lizarralde 1995), capture of women (Otterbein 2000; Walker and Bailey 2013), greater reproductive success for participants (Chagnon 1988; Glowacki and Wrangham 2015), and other material and cultural rewards oriented particularly to male participants (Glowacki and Wrangham 2013; Glowacki et al. 2018).

Later on, Kim and Kissel state, “As we will discuss, the notion that men are more violent than women is a commonly held belief. However, this is not necessarily true” (2018:127). Yet, as far as I can tell, they never revisit this idea again. Although some research has indicated little evidence of sex differences in lower-cost physical aggression (Archer 2000), in every known society males commit significantly more lethal violence than females do, as seen across hunter-gatherer societies (Fry and Söderberg 2013) and nation-states alike, with males committing more than 95% of homicides globally (UN Office of Drugs and Crime 2013). Kim and Kissel fail to address a core ethnographic fact: warfare is a highly gendered behavior, almost exclusively male, and biological sex differences are a key factor in understanding the emergence of warfare.

To conclude, I appreciate Dunsworth’s focus on developmental factors and her reference to hominid ancestry in trying to understand patterns of sexual dimorphism in humans. However, her argument that male competition has been overemphasized across hominids more generally seems misplaced in light of the data provided by Plavcan, and even if recent sexual selection has not been a strong force in contributing to sex differences in height among humans, competition between males has very clearly been an important facet of human societies throughout our history.

Conceptual Issues with Evolutionary Psychology: The Case of the ‘Mate-Killing Module’

In his book The Dangerous Passion (2000), evolutionary psychologist David Buss provides “an evolutionary explanation for mate killing,” (pg. 120) based on earlier work with his colleague, evolutionary psychologist Joshua Duntley. Buss writes that, “Duntley and I argue that men have evolved a mate-killing module, a psychological mechanism whose function is not threat or deterrence, but rather the literal death of a mate.” (pg. 122).

Now, at first glance, this may seem like a very extreme claim (and to be clear, I think it is), however when we read Buss’s reasoning, we can see the proposal is not that men have a kind of unchecked urge or drive to murder their partners, but rather that they have a “psychological mechanism” that leads them to do so in certain contexts, “According to this theory, over the long course of human evolutionary history, it has been reproductively advantageous for men in some circumstances to kill an errant partner, especially when the finality of her departure sinks in.” (pg. 123). But if indeed this is a behavior that men are expected to engage in only in select, specific contexts, it is hard to see the utility of the unspecified and overly general ‘mate-killing module’ explanation. Buss and Duntley are, without appearing to realize it, actually making a behavioral ecology argument about different socioecological contexts incentivizing different fitness strategies, and then adding on top of that a single ‘psychological module’ that supposedly explains this behavior across these contexts.

This is important because evolutionary psychology as a discipline was founded on opposition to certain key dimensions of human and animal behavioral ecological thinking, which generally considers behavior as reflecting fitness maximizing strategies that vary substantially depending on the socioecological environment. For example, one of the founders of evolutionary psychology, Donald Symons, in an influential 1989 paper that laid the intellectual groundwork for the discipline, writes that, “In DP [‘Darwinian psychology’], human activities in environments differing radically from the EEA [Environment of Evolutionary Adaptation] are as informative as human activities in more traditional settings, even though the former are likely to depart radically from the path of fitness maximization.”

In other words, if men really evolved a ‘mate-killing module’, then why is it that men are only supposed to express it in specific, fitness relevant and (supposedly) adaptive, contexts? Evolutionary psychologists often contest the socioecologically-contingent fitness maximization logic in other domains and emphasize the primacy of ‘psychological mechanisms’, which may be adaptive or mismatched in different environments. Buss, in the most recent volume of his textbook Evolutionary Psychology (2016), writes that,

An important point to keep in mind is that a mechanism that led to a successful solution in the evolutionary past may or may not lead to a successful solution now. Our strong taste preferences for fat and sugar, for example, were clearly adaptive in our evolutionary past because fat from meat and sugar from ripe fruits were valuable and scarce sources of calories. Now, however, with pizza places selling pies and sugar-laden soft drinks on every street corner, fat and sugar are no longer scarce resources. Thus, our strong taste for such substances now causes us to overconsume fat and sugar, which can lead to clogged arteries and heart attacks and thereby hinder our survival. This can be called a mismatch between ancestral and modern environments (see Li, van Vugt, & Colarelli, 2017). We will be examining other evolutionary mismatches later in the book, but the central point is that evolved mechanisms exist in the forms that they do because they led to success on average during the period in which they evolved [emphasis added] (pg. 44).

Note the final sentence: “success on average during the period in which they evolved,”—can one plausibly argue that killing one’s partner was, on average, adaptive? Putting aside the lack of real utility of the ‘mate-killing module’ explanation, as well as the lack of any evidence for it whatsoever, at the level of the brain, or the genetic level, or otherwise, let us consider the specific circumstances Buss and Duntley have in mind for when this ‘mate-killing module’ is supposed to express itself (see pgs. 122-123).

Buss writes, “First, in a polygynous mating context, where a man might have several wives, killing one wife as a result of an infidelity or defection could prevent other wives from cheating or leaving.”

Maybe. However, could this not just as easily induce other wives to attempt to flee when opportunity arises, or join up with an alternative male protector, in ways that hurt this hypothetical man’s fitness rather than help? —to say nothing of possible reprisal from the wife’s kin. In any case, this logic seems very dubious in most circumstances. Next,

Second, in some cultures, a man’s reputation would have suffered so extensively as a result of a wife’s infidelity that killing her would be the only means of salvaging lost honor. Killing an unfaithful wife sometimes restores a man’s honor. As Daly and Wilson note, “Not infrequently, men salvage some of their lost honor by killing an unchaste wife . . . and the male seducer. Shrinking from such vengeance may even add to their dishonor.”

I don’t think this is necessarily wrong, but note, even more so than with the polygyny example, we are dealing with a clearly *learned and culturally contingent* phenomena. We’re talking about particular cultures oriented around certain conceptions of honor which are socially learned and historically contingent. In this case the ‘mate-killing module’ is very obviously unnecessary to explain this. Further, it is not necessarily clear that such behaviors would increase one’s reproductive success, even if they may serve to in some cases protect one’s status (at least among the men or perhaps their lineage) within their society. They might, but just as with the polygyny example this hasn’t actually been demonstrated, Buss and Duntley are operating on the basis of plausibility rather than actual solid evidence. Let’s consider the next few examples together in relation to Buss and Duntley’s conclusion:

Third, a sexual infidelity may have inflicted such a severe cost on a man in the currency of paternity uncertainty and the associated misdirection of his investments, that killing the woman may have been a viable means of stanching the costs. If she is pregnant with another man’s child, he also hurts his rival’s reproductive success.

Our fourth argument hinges on the fact that one of the major triggers of mate killing is an irrevocable loss of the relationship. When a woman finally convinces her partner that she’s leaving for good, the loss may be so substantial that it pushes the man over the edge into entertaining homicidal thoughts.

The final end of the relationship, in sum, historically may have put the man in triple jeopardy in the currency of reproduction. He lost entirely his access to her reproductive capabilities. He suffered severe and possibly irreparable reputational damage as a result of the loss. And if the woman was at all desirable, it was likely that she would remarry, so that a man’s loss would have been his rival’s gain. His same-sex rival benefited in direct proportion to the original man’s loss (pg. 123).

I do think these last two examples are contexts where men across cultures do seem to be more likely to kill their spouses (see Daly & Wilson, 1988), although again it is unclear why the ‘mate-killing module’ is a necessary or useful explanation for this phenomena. Buss quotes evolutionary psychologists Margot Wilson and Martin Daly as arguing instead that such spousal homicides are more an unfortunate and often unplanned byproduct of mate-guarding strategies, ““Men . . . strive to control women . . . women struggle to resist coercion and to maintain their choices. There is brinkmanship and risk of disaster in any such contest, and homicides by spouses of either sex may be considered slips in this dangerous game.”” (pg. 121). This explanation need not necessarily be correct either, though notably it is more consistent with the limited data available and is not reliant on currently unsupported and likely unsupportable claims about an evolved ‘mate-killing module’.

Let me now give an example from a small-scale society of cultural incentivizes playing a role in a man killing his wife, as well as a specific case from the same society more in line with Daly and Wilson’s perspective than Buss and Duntley’s. In his work Naven (1936) discussing the Iatmul forager-horticulturalists of New Guinea, anthropologist Gregory Bateson writes that,

In general the fighting and killing was confined to the killing of foreigners, i.e. members of other villages, especially of villages against whom a feud existed. But even this rule was not too strictly interpreted; a woman, married into the village, might for purposes of head-hunting be considered a foreigner. I even came across one case in which a man wore a tassel for killing his own wife in revenge for a kill accomplished by members of the village from which she had come.

In one example of a man killing his wife, recounted to him by a man who was the classificatory brother of the victim, Bateson writes,

Laindjin killed Tualesh, his wife. She was a member of Wolgem clan and (therefore) my (classificatory) sister. Tualesh went to get water in a long bamboo. When she returned Laindjin said, “Why were you so long?” She said, “I was not long. What are you thinking?” Laindjin said, “I was only asking," and she said, “Other men don't ask, but you do. You are jealous.” Laindjin said, “Yes, I know how women carry on.” Then he jumped up and seized the water bamboo and bashed her with it and she died. They made incisions in her skin with a bamboo knife and used spells, but they could not cure her. Then Laindjin wept.

Notably, after this killing Laindjin was attacked by members of his wife’s clan, in a strike against his household which left his brother’s son injured. Afterwards, Laindjin further had to provide restitution to his wife’s clan in the form of, “one mother-of-pearl crescent, one tortoise-shell arm band, two Conus shell necklaces and three Turbo shells, in a string bag which was hung up in the Wolgem ceremonial house.”

There are two key points to keep in mind here which, while specific criticisms of the ‘mate-killing module’ hypothesis, are also worth thinking about when considering other proposed ‘psychological adaptations’, particularly ones that are expected to manifest only in particular socioecological contexts:

1) Buss and Duntley do not provide any evidence that mate killing increases a man’s reproductive success in any of the contexts that they are discussing. They simply propose that it does, on the basis of plausibility alone (see Gould & Lewontin, 1979). One way of evaluating this would be to look across different societies and show that men who do kill their wives have more children that survive to adulthood than men who don’t kill their wives. This at least would provide some support for the idea that killing one’s mate can be part of an effective fitness strategy, however…

2) Even if it were shown that killing one’s mate does increase a man’s reproductive success in certain socioecological contexts, in other words, that it can potentially be an adaptive behavioral strategy, this does not show that it is a behavior resulting from a specific psychological adaptation, such as the proposed ‘mate-killing module’.

To provide an example of how to investigate these kind of questions more carefully, anthropologists Richard Wrangham and Luke Glowacki’s work on the evolution of warfare offers a salient contrast to Buss and Duntley’s more simplistic adaptationist thinking. In their paper comparing patterns of intergroup conflict among chimpanzees and nomadic hunter-gatherer societies, Wrangham and Glowacki find some key similarities. As I wrote in a previous piece,

Glowacki and Wrangham have…shown that intergroup violence in both chimpanzees and among nomadic hunter-gatherers is often undertaken tactically; aggressors are more likely to attack when they have a numerical advantage and can ambush outsiders who encroach on their territory. Indiscriminate use of violence is not favored, but the tactical use of violence can represent a biologically and/or culturally adaptive strategy.

Importantly, Wrangham and Glowacki appropriately conclude their article by writing that,

In sum, nomadic hunter-gatherer warfare tends to conform to the chimpanzee model, but whether humans have also evolved species-specific adapations for war is uncertain. The chimpanzee model therefore appears to be a useful starting point for analyzing the cultural evolution of warfare, and possibly its biological evolution as well.

Wrangham and Glowacki have been carefully exploring this topic in a series of papers over the last few years. They have shown how cultural rewards are often key in incentivizing participation in warfare in small-scale societies. They also found that among the Nyangatom pastoralists of East Africa, elders who participated in more stealth raids against rival communities in their youth had more wives and surviving children. Even where they point to the role of various potentially evolved psychological mechanisms in warfare, they caution against boiling such a complex behavior down to a single propensity,

This strategy [warfare] is not the result of a single “instinct” for war, but is instead an emergent property resulting from evolved psychological mechanisms (such as xenophobia and parochial altruism). These mechanisms are sensitive to ecological and social conditions, such that the prevalence and patterns of warfare vary according to subsistence strategies, military technology, cultural institutions, and political and economic relations.

Napoleon Chagnon also warned against boiling complex behaviors down to particular ‘instincts’, writing that, “one cannot account for the engagement [in warfare] by citing psychological variables such as anxiety or in terms of innate aggression. To do so is to confuse effects with functions and reduce cultural phenomena to bio-psychological variables.”

Evolutionary psychologists are sometimes critical of human and animal behavioral ecologists for giving insufficient attention to the role of psychological factors (particularly putative psychological adaptations) in complex behavior, without recognizing that in many cases this is the only responsible way to handle the limited data available to behavioral scientists, and avoids unnecessary reductionism. As anthropologist E.A. Smith writes,

HBEs [Human Behavioral Ecologists] tend to focus on explaining behavioral variation as adaptive responses to environmental variation; they assume that this adaptive variation (facultative behavior, phenotypic response) is governed by evolved mechanisms that instantiate the relevant conditional strategy or decision rule. This assumption, which takes a “black-box” approach to the actual mechanisms involved, is part of what Grafen (1984) terms the phenotypic gambit. This means taking a calculated risk to ignore the (generally unknown) details of inheritance (genetic or cultural), cognitive mechanisms, and phylogenetic history that may pertain to a given decision rule and behavioral domain in hopes that these don’t matter to the end result.

The important thing to remember is that complex behaviors such as these emerge from innumerable factors sensitive to particular contexts. Boiling mate-killing, or warfare, or numerous other complex behaviors down to a single ‘psychological adaptation’ is not only often unjustified in light of the lack of neurological and genetic evidence available to evaluate such claims about adaptations, but it actually obscures the more immediately relevant socioecological factors that actually let you make predictions and test hypotheses about the phenomena in question.

Postscript

My friend anthropologist Cody Moser has a nice post on evolutionary psychology where he offers some constructive suggestions for investigating evolutionary psychology hypotheses:

1. Emphasizing strict, cross-cultural and developmentally or biologically-constrained patterns over broader ones

2. Moving to identify genetic mechanisms associated with these patterns

3. Identifying modules associated with these mechanisms (in this case, coupling with the ongoing connectome project could be fruitful) OR moving away from modules entirely and moving to a broader neurophysiological or hormonal-affect state models

4. Most importantly, emphasizing the room that spandrels might play in our current behaviors or how developmental scaffolds may build upon more baseline-level EP module-esque systems to build higher-order emergent properties (I think Tooby & Cosmides “computational theory of mind” partially covers this, but even so this seems to be less emphasized in EP than its other aspects)

Book Review: Not Born Yesterday by Hugo Mercier

There are two core arguments put forward in Not Born Yesterday, the latest book by cognitive scientist Hugo Mercier. The first argument, one which I believe has a great deal of truth to it, is hinted at by the title—the gullibility of the average person has been greatly exaggerated. Human beings are discriminating figures, capable of evaluating contradictory messages from self-interested interlocuters and protecting themselves from the deceptions of others. That seems right to me.

The second argument, far less persuasive in my view, is that this epistemic vigilance makes deception from others relatively rare and unsuccessful for persuasive purposes. Here’s a nice quote from the book which summarizes Mercier’s perspective,

We aren’t gullible: by default we veer on the side of being resistant to new ideas. In the absence of the right cues, we reject messages that don’t fit with our preconceived views or preexisting plans. To persuade us otherwise takes long-established, carefully maintained trust, clearly demonstrated expertise, and sound arguments.

I think the first sentence is broadly right, but as I will explain, the next two sentences have a number of assumptions baked into them which lead Mercier to underestimate how effective self-interested manipulation and deception can be. Mercier says, “The multiple mass persuasion attempts witnessed since the dawn of history—from demagogues to advertisers—are no proof of human gullibility. On the contrary, the recurrent failures of these attempts attest to the difficulties of influencing people en masse.”

According to Mercier, national propaganda might signal the power of the state, for example, but it is unlikely to actually convince people of its message. However, notice the problem of scale here—we did not evolve in contexts of frequent messaging from competing strangers through technologically advanced mass media. Whether or not various mass persuasion endeavors were effective at persuading the average citizen in a modern state society seems to me to have little bearing on the question of the evolution of our persuasive or deceptive abilities. It does, however, offer some support for the argument about people not necessarily being easily swayed or intrinsically gullible by just any communicative signal put to them.

The arguments about gullibility in the book tend to be more carefully considered than the ones about the lack of deception, and Mercier sometimes finds himself having to make rhetorical sacrifices to the latter argument in order to stay on firm ground with the former. For example, Mercier writes that,

Even when mass persuasion bears on the message itself, it can reach its goal without entailing any gullibility on the part of the audience. Many of the products we buy exist in virtually identical versions—different brands of soda, toothpaste, detergent, cigarettes, milk. In these conditions, it is only normal that our minds should respond to minor nudges: the position on the shelf, a tiny discount, or even an appealing ad. Shifts between these essentially identical products may be of no import for consumers, while making a huge difference for companies. Some ads can be cost-effective without any genuine persuasion having taken place.

I think Mercier is right in noting that just because people seem to respond to minor social cues that doesn’t make them gullible, and it can be perfectly in their interests, or at least non-costly, to do so, but this fact also clearly conveys opportunities for manipulation or deception in the space of “virtually identical” products, with Mercier forthrightly saying it is, “only normal that our minds should respond to minor nudges: the position on the shelf, a tiny discount, or even an appealing ad,” in such contexts.

Mercier writes that,

Deceit is cognitively demanding: we have to think of a story, stick to it, keep it internally coherent, and make it consistent with what our interlocutor knows. By contrast, negligence is easy. Negligence is the default. Even if we are equipped with cognitive mechanisms that help us adjust our communication to what others are likely to find relevant, making sure that what we say includes the information our interlocutor wants or needs to hear is difficult.

I don’t think this is necessarily wrong although trying to distinguish intentional deceit from negligence can be tricky—someone can be providing you with self-serving information despite knowing it’s false, or be providing you with self-serving information and simply not caring whether it is false—and there is surely a whole spectrum of variation with different deceivers exhibiting varying degrees of awareness of how accurate or not the information they communicate may be. The example Mercier gives, instead of clarifying things, compounds the difficulty of differentiating intentional deception from negligence. He writes that,

But deceit is not the only, or even the main, danger of communication. Imagine that you are looking to buy a used car. The salesman might lie to you outright: “I have another buyer very keen on this car!” But he is also likely to give you misguided advice: “This car would be great for you!” He might believe his advice to be sound, and yet it is more likely to be driven by a desire to close the sale than by a deep knowledge of what type of car would best suit your needs. Now you ask him: “Do you know if this car has ever been in an accident?” and he answers “No.” If he knows the car has been in an accident, this is bad. But if he has made no effort to find out, even though the dealership bought the car at a suspiciously low price, he is culpable of negligence, and this isn’t much better. In the case at hand, whether he actually knew the car had been in a crash, or should have known it likely had been, makes little difference to you. In both cases you end up with a misleading statement and a lemon.

Note that in this example there are still explicit advantages to knowing what you don’t know. The negligent car salesman who happens to not know the history of some of his cars is still at a competitive disadvantage with the car salesman who knows what he doesn’t know and how to make strategic use of that fact, adjusting his behavior accordingly. So instead of clearly demonstrating negligence is a more important factor than deception, this example makes it seem to me more of a spectrum of variation than necessarily competing explanations for social manipulation, where asymmetries of knowledge are taken advantage of by self-interested parties who use various strategies, including negligence and intentional deception, to do so.

Other examples Mercier gives also indicate deception has been historically more important and prevalent than Mercier suggests it has been. Mercier begins the book with an anecdote about being swindled by a con man pretending to be a doctor,

AS I WAS WALKING BACK from university one day, a respectable-looking middle-aged man accosted me. He spun a good story: he was a doctor working in the local hospital, he had to rush to some urgent doctorly thing, but he’d lost his wallet, and he had no money for a cab ride. He was in dire need of twenty euros. He gave me his business card, told me I could call the number and his secretary would wire the money back to me shortly. After some more cajoling I gave him twenty euros. There was no doctor of this name, and no secretary at the end of the line. How stupid was I? And how ironic that, twenty years later, I would be writing a book arguing that people aren’t gullible.

Later on Mercier offers an explanation for this event, in conjunction with his discussion of the first person identified to be a con man. Mercier writes that,

The first man to be called a con man was Samuel Thompson, who operated around 1850 in New York and Philadelphia. He would come up to people, pretend to be an old acquaintance, and remark on how people did not trust each other anymore. Making his point, he would wager that the mark wouldn’t trust Thompson with their watch. To prove him wrong, and to avoid offending someone who appeared to be a forgotten acquaintance, some people would give Thompson their watch, never to see him or their watch again. Thompson relied on his “genteel appearance” (a coarse cue indeed) to pressure his victims: they might not have trusted him altogether, but they feared a scene if they blatantly distrusted someone of their own social standing. This is how the fake doctor from the introduction got me to give him twenty euros. Once you accept the premise that someone is who they say they are, a number of actions follow logically: had that person been a real doctor, I should have been able to trust him with the money. And rejecting the premise, saying to someone’s face that we think they are a fraud, is socially awkward.

In my view this paragraph actually helps us understand why manipulation and deception are common, not rare. It is clearly embedded in the assumptions Mercier conveys here about doctors, and the description of why Thompson and the fake doctor’s deceptions worked—people are often reluctant to question or challenge widely agreed-upon social norms and conventions, understandably fearing a loss of status and reputation: in every society there are individuals who are willing and able to take advantage of this fact to gain benefits for themselves. This is a topic I have discussed at length on this website.

One thing evolutionary accounts of human history often miss is the commonality of sorcerers and secret societies throughout our history, attesting to the importance of social manipulation and how self-interested parties take advantage of asymmetries of information. Historian Daniel Jütte writes that, “In contemporary Western societies, frequently invoked categories like “the public” and “openness” are critical for shaping our ideals about how people should live together and how knowledge should be produced. As a result, we greatly underestimate the importance of secret forms of knowledge both in the premodern world and in contemporary non-Western societies.”

As I discussed previously, Mande blacksmith clans in West Africa offer one salient example of the importance of secret forms of knowledge in smaller-scale societies. Anthropologist Patrick McNaughton writes that,

Nearly everyone believes that members of these special clans possess a mysterious spiritual power that underpins occult practices and makes the people possessing it potentially dangerous. These powers go well beyond the practice of the clan's special trade, but they are also considered essential to anyone who takes it up. Often members of these clans go to great lengths to nourish a belief in their power among the rest of the population. Indeed, they generally believe in it themselves. Furthermore, they say they are born with much of this power. It is part of their heritage and one of the things that makes them so different from everyone else. That too creates a profound handicap for any outsider who might want to earn a clan's special trade.

I tend to think human’s ‘epistemic vigilance’, far from disincentivizing social manipulation and deception, has contributed to the evolution of more elaborate and extravagant forms of them. In his paper on the cultural evolution of shamanism, anthropologist Manvir Singh writes,

I propose that shamanism is a suite of practices developed through cultural evolution to convince observers that an individual can influence otherwise uncontrollable out-comes. In particular, the shaman is an individual who violates intuitions of humanness to convince group members that he or she can interact with the invisible forces who control unpredictable, important events.

Singh, like Mercier, disagrees with simple narratives that focus narrowly on viewers gullibility to charlatans, but Singh’s framework demonstrates how competition among shamans, effectively the ‘doctors’ of many small-scale societies, leads to more elaborate performances to persuade viewers of supernatural power.

“When outcomes of uncertainty are controlled by invisible forces, cultural selection will favor individuals who claim special abilities of interacting with those forces.” Figure from ‘The cultural evolution of shamanism’ (2017) by Manvir Singh.

Now, let us revisit Mercier’s conclusion—"In the absence of the right cues, we reject messages that don’t fit with our preconceived views or preexisting plans. To persuade us otherwise takes long-established, carefully maintained trust, clearly demonstrated expertise, and sound arguments.”

Another way of putting it is, people are credulous to views that do fit their “preconceived views or preexisting plans,” and this leaves room for deceivers to take advantage of such biases and incentives. Further, if individuals provide some signal that seems to indicate their impressive power, or knowledge, or status, people do exhibit an increased willingness to believe them.

I think Mercier is right about people not being intrinsically gullible, and having a variety of cognitive tools at their disposal for evaluating trustworthiness, but the conditions for people to develop and perpetuate costly beliefs which may cause them harm, or beliefs that benefit some people at the expense of others and are promoted through self-interested deception, are much more common than conveyed in the book.

Contests Between Equals: Men and Beasts

Man…does not stand completely apart from animals. Both are N!adima's [the supreme deity’s] creatures and are subject to his anger and caprice and, to about the same extent, to the vagaries of the natural environment. Hunting is part of the battle for survival, and the hunted animal is an adversary in the contest of the hunt, not something that exists specially for man to prey upon – George Silberbauer, The Gwi Bushmen, 1972.

“An Assiniboine running a Buffalo. Drawn by an Assiniboine warrior and hunter.” From Indian Tribes of the Upper Missouri (1961) by Edwin Thompson Denig.

It is easy to focus on the aftermath of a hunt—an event concluded with death and consumption; an aggressive act of destruction, dismemberment, and digestion. Despite his visceral actions, the successful hunter is often aided in his pursuit by his respect for the intelligence and abilities of his prey. Anthropologist Lev Sternberg writes of the Gilyak fisher-foragers of Siberia that, "Every animal is in point of fact a real being like a man, nay a Gilyak such as himself, but endowed with reason and strength which often surpasses those of man.” Anthropologist George Silberbauer gives a detailed discussion of perceptions of animal intelligence and behavior among the G/wi foragers of the Kalahari, writing that,

animal behavior is perceived as rational and purposive and directed by motives based on values that are either held by the G/wi themselves or by other peoples known to them or are negations of such values. The motivational and value systems of animals are not isomorphic in all respects with G/wi and other human systems but are empathetically modified to fit the perceived circumstances of the animals themselves. Each species is credited with characteristic behavior, which is governed by its kxodzi (customs), and each has its particular kxwisa (speech, language). Protocooperating (i.e., mutually beneficial) species and even some that are hostile to one another can understand one another's language, and some animals can even understand a certain amount of G/wi. Man, too, can understand a limited amount of the speech of some species, for example, the alarm cries of birds. Baboons, the most versatile of animal polyglots, eavesdrop on G/wi hunters and pass on the plans of the hunters to the intended prey animals. This is not altruism but is caused by the baboons' legendary love of trickery and teasing.

The special capabilities of some animals are believed to have been arrived at by rational thought and then institutionalized as elements of the species' kxodzi (customs) after having been passed on by the discoverers or inventors in that population. (For instance, the ability of the penduline tit, Anthoscopus minutus, to manufacture feltlike nesting material from the awns of grasses.) Such capabilities are compared with those that man has developed in the process of devising means of meeting environmental pressures. Some species possess knowledge that transcends that of man; the bateleur eagle, Terathopius ecaudatus, for instance, knows when a hunter is to be successful and hovers above him, thus acting as an omen of sure success (but not, apparently, as a warning to the prey). Several animals are believed to be able to foretell the extent of wet-season rains and the location of the best falls and to plan their annual behavior cycle accordingly. These animals are seen as having rather critical limits of tolerable error. If they are to reproduce successfully, they must time their activities to gain the greatest benefit from the rains when they do come. They are believed to have a more sensitive perception of how the rains are developing and can thus furnish the discerning observer with more accurate information than his own less finely tuned senses can gather. The duiker, Sylvicapra grimmia, practices sorcery against his animal enemies and even against conspecific rivals, and some steenbok, Raphicerus campestris, are thought to possess a magical means of protecting themselves from a hunter's arrows.

Name taboos associated with animals, particularly those sought as prey, are not uncommon across forager societies. Of the Batek of Malaysia, anthropologist Lye Tuck-Po writes that, “the name of the animal being hunted cannot be uttered from the start to the end of a hunt. Before the kill, uttering the name will alert the prey to its oncoming fate. After the kill, uttering the name might provoke revenge, or show disrespect to the animal,” while among the Tlingit foragers of Alaska, missionary Livingston French Jones writes that,

It is believed that all animals understand human speech. For this reason natives are careful what they say about them not only in their presence, but at any time; for they have some mysterious way of hearing all said about them, and if evil or boastful things are said, the creature maligned is sure to take offence, and in time will surely harm the speaker. A young man who was subject to epileptic fits, while in one of them fell off the deck of a boat and was drowned. It was said that when he was a child he spoke unkindly to some little fishes, and this was his punishment for it. A young man swore at some mountain sheep which he was hunting because they were in a difficult place to reach. In his effort to reach them a snowslide came down and buried him and he perished. The natives believe that he met with this death because he was disrespectful to the sheep.

Rituals and norms of conduct associated with the hunt can attest to the healthy respect and sense of dignity accorded animals by the hunter. Among the Guayaki-Ache hunter-gatherers of the Amazon, anthropologist Pierre Clastres writes,

As for the animals, there are certain rules of courtesy to be observed toward them. When the hunter kills them, he must also salute them; he arrives in the camp, his game hanging over his shoulder, which is nobly spotted with blood, and he puts it down and sings in its honor. In this way, the animal is not simply a neutral piece of food; if it were reduced to that, the other members of its species might get angry and not allow themselves to be shot anymore. Hunting is not simply a matter of killing animals; you owe them something, and this debt is paid when you bring the animals you have killed back to life by talking about them. You thank them for letting themselves be killed, but their common names are never mentioned. So that brevi, tapir, is called morangi, and kande, the little peccary, is called barugi. You have to be sly with the animals, you must pretend to be talking about someone else; by fooling the game this way, you somehow annul man's aggressiveness and wipe out the fatal act. The hunter's chanting seals the secret agreement between men and animals. The kybuchu are also taught this: to live in the forest one must avoid excess and respect the unity of the world in order to make sure it continues to be generous.

This reference to “man’s aggressiveness” and “the secret agreement between men and animals” dovetail with accounts of male secrecy and rituals oriented around hunting found in some forager societies. Anthropologist Lorna Marshall writes that, “!Kung society accords the hunt great importance and, for the !Kung, hunting is entirely a male affair. Women are wholly excluded from hunting and from taking part in the several hunting rituals that the men perform.”

!Kung hunters. From Lee (1979)

Anthropologist Vladimir Jochelson writes that among the Yukaghir foragers of Siberia,

When the autopsy of a killed animal reveals under the hide a cartilaginous hardening, obviously of a pathological character, the Yukaghir say that that is where the pe'jul [guardian spirit of the animal + good fortune] is located. Usually such growths are found on the neck, shoulder or chest of the reindeer. The Yukaghir dry them and carry them about in small leather bags, as amulets. Naturally enough, views like the above about the animals necessary for life, led to a cult of the animal, that is, to a religious attitude towards. The killed animal is treated like a dear and honored deceased friend. Its face is covered. Girls who have reached maturity, menstruating women and women after child-birth must not go after the meat of the killed animal, for they might offend it by their unclean condition. A girl must not leave the house when her brothers are hunting.

The ethos of masculine competition embodied in the hunt takes its most vivid form in the solitary, ritualized confrontations men sometimes seek out with the most dangerous animals in their environment. Missionary Antonio Colbacchini described the individual jaguar hunt conducted among the Bororo of Brazil, sometimes done by a man after the death of a relative or a friend, or other circumstances, writing that,

They often encounter the adugo, the terrible spotted ounce. The Indian does not retreat before the terrible feline, but immediately shoots a dart at it, trying to hit the heart. Rarely is the first stroke so accurate that it makes the animal fall. Enraged by the pain, it attacks the savage, who immediately holds the bow with both hands and puts it horizontally in front of his eyes. When the beast approaches the hunter, it rears up on its hind legs and, thus upright, tries to hurt him with the front ones and shred him with its teeth. As soon as the feline stands upright, the Indian, with his arms outstretched, pushes forward his bow, on which the ounce leans, giving horrible roars. And there, firm, with muscles tensed, eye to eye, the man and the beast remain a long time, one trying to overcome the other with muscular pressure. These must be unforgettable moments for the whole life, if the savage reaches victory. Finally the animal slackens the pressure and retreats one step to retake the offensive; the man, with prodigious dexterity, places a new arrow in the bow and wounds the ounce a second time. With renewed ferocity it tries to jump over the Indian, who is immediately ready, as before, to stop it from making the jump. And thus the struggle goes on, until, weakened, the bleeding beast falls, and the Indian takes advantage of that to multiply the arrows and kill it. Unfortunate for the hunter if he hesitates. A heavy blow on the forehead or on the shoulders would make him the prey of the jaguar; many Indians die this way. An Indian had a purulent wound on the upper lip, at the base of the nose, and had become hunchbacked in one of those encounters; he came out the loser, but other companions, who killed the beast, approached in time. It had, however, already made a lesion in the vertebral column and on the face of the hunter, leaving him hunchbacked and disfigured. Ukeiwaguuo, too, in the vicinity of the aldeia, was attacked by an ounce. His screams attracted other Indians, who ran to help him. He, however, defended himself courageously.

Anthropologist Dominique Desson writes among foragers in the Kodiak archipelago near Alaska that,

Whaling on Kodiak...was a solitary occupation, a one-to-one confrontation between man and beast. By wearing the closed crown hat in a ritualized context (the hunt proper) the whaler was transformed symbolically into a killer whale (symbol of hunting and killing prowess) and whaling thus became the ritualized warfare between equals.

Kodiak whaling mask. From Desson (1995)

The practice of hunting for many forager men is not only essential for survival but conveys the highly-esteemed spirit with which they regard many of the animals they pursue. It has always been a fact of human existence, that animals must die so we may live. Across many forager societies, these violent acts of destiny tend to be managed with as much respect and honor as men are capable of.